Mathematically optimal position sizing based on your edge.
Kelly % = W − (1−W) / R, where W = win probability and R = win/loss ratio. The Kelly formula maximizes the geometric growth rate of your bankroll in the long run.
In practice, most professional traders use Half Kelly (50% of full Kelly) or less. Full Kelly is mathematically optimal but causes extreme account volatility. Kelly is highly sensitive to input accuracy — be conservative in your estimates.